On 18/04/17 22:28, Anthony D'Atri wrote: > I get digests, so please forgive me if this has been covered already. > >> Assuming production level, we would keep a pretty close 1:2 SSD:HDD ratio, > > 1:4-5 is common but depends on your needs and the devices in question, ie. > assuming LFF drives and that you aren’t using crummy journals. > >> First of all, is this even a valid architecture decision? > > Inktank described it to me back in 2014/2015 so I don’t think it’s ultra > outré. It does sound like a lot of work to maintain, especially when > components get replaced or added. > >> it should boost performance levels considerably compared to spinning disks, > > Performance in which sense? I would expect it to boost read performance but > not so much writes. > > I haven’t used cache tiering so can’t comment on the relative merits. Your > local workload may be a factor. > > — aad
As it happens I've got a ceph cluster with a 1:2 SSD to HDD ratio and I did some fio testing a while ago with an SSD-primary pool to see how it performed, investigating as an alternative to a cache layer. Generally the results were as aad predicts - read performance for the pool was considerably better, almost as good as a pure SSD pool. Write performance was better but not so significantly improved, only going up to maybe 50% faster depending on the exact workload. In the end I went with splitting the HDDs and SSDs into separate pools, and just using the SSD pool for VMs/datablocks which needed to be snappier. For most of my users it didn't matter that the backing pool was kind of slow, and only a few were wanting to do I/O intensive workloads where the speed was required, so putting so much of the data on the SSDs would have been something of a waste. -- Richard Hesketh
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